Long Term Evolution (LTE) is an evolution of Universal Mobile Telecommunications System (UMTS), standardized by an international standardization body, 3rd Generation Partnership Project (3GPP). The configuration of an LTE system is illustrated in FIG. 1.
FIG. 1 is a view referred to for describing the configuration of an LTE system.
The LTE system may be divided largely into an Evolved UMTS Terrestrial Radio Access Network (E-UTRA) and an Evolved Packet Core (EPC). The E-UTRAN includes UEs and evolved Node Bs (eNBs). A UE is connected to an eNB via a Uu interface and one eNB is connected to another eNB via an X2 interface. The EPC includes a Mobility Management Entity (MME) responsible for control-plane (C-plane) functions and a Serving GateWay (S-GW) responsible for user-plane (U-plane) functions. An eNB is connected to the MME via an S1-MME interface and an eNB is connected to the S-GW via an S1-U interface. These two interfaces are collectively called an S1 interface.
For the Uu interface being an air interface, a radio interface protocol stack is defined. The radio interface protocol stack horizontally includes a PHYsical (PHY) layer, a data link layer, and a network layer and vertically includes a U-plane for user data transmission and a C-plane for control signaling. Based on the lowest three layers of the Open System Interconnection (OSI) reference model known in communication systems, this radio protocol stack can be divided into Layer 1 (L1) including a PHY layer, Layer 2 (L2) including a Medium Access Control/Radio Link Control/Packet Data Convergence Protocol (MAC/RLC/PDCP) layer, and Layer 3 (L3) including a Radio Resource Control (RRC) later. These layers are defined in pairs between a UE and an Evolved UTRAN (E-UTRAN), for data transmission via the Uu interface.
Now a description will be given below of a Long Term Evolution Advanced (LTE-A) system.
LTE-A is a system developed from LTE to meet the 4th Generation (4G) mobile communication requirements, that is, IMT-Advanced requirements recommended by the International Telecommunication Union-Radio communication sector (ITU-R). The 3GPP which developed the LTE system standard is now actively working on standardization of the LTE-A system.
Major technologies added to the LTE-A system are carrier aggregation for extending a used bandwidth and flexibly using the bandwidth and use of relays for supporting group mobility and enabling user-centered network deployment.
FIG. 2 is a view referred to for describing the concept of a mobile communication system to which relays are added.
A relay or RN relays data between a UE and an eNB. Because communication is not conducted actively between a UE and an eNB that are apart from each other by a long distance in the LTE system, a network node called an RN was introduced between a UE and an eNB to avert the problem in the LTE-A system. An eNB managing an RN is called a Donor eNB (DeNB) and a new interface between the RN and the DeNB is defined as a Un interface, distinguishably from a Uu interface between a UE and a network node. FIG. 2 conceptually illustrates an RN and a Un interface.
The RN functions to manage UEs on behalf of the DeNB. That is, the RN is perceived as the DeNB to the UEs. Thus, the Uu interface between the UE and the RN still uses the conventional Uu interface protocols, MAC/RLC/PDCP/RRC.
From the perspective of the DeNB, the RN is perceived as a UE or an eNB depending on circumstances. That is, when the RN initially accesses the DeNB, it performs random access as done by a UE, because the DeNB is not aware of the existence of the RN. Once the RN is connected to the DeNB, the RN operates like an eNB that manages its connected UEs. Accordingly, Un interface protocols should be defined to include a network protocol function in addition to the functions of the Uu interface protocols. For the Un protocols, the 3GPP is currently discussing functions to be added to each protocol layer or functions to be changed in each protocol later, based on the Uu protocols such as the MAC/RLC/PDCP/RRC protocols.
Hereinbelow, a description will be given of an inband RN and an outband RN.
When a Un interface between a DeNB and an RN and a Uu interface between the RN and a UE operate at different frequencies, the RN is called an outband RN. Since a frequency should be allocated to the Un interface, additional cost is caused for the frequency allocation and operation of the RN. To overcome the shortcomings of the outband RN, the concept of an inband RN was introduced additionally.
FIG. 3 is a view referred to for describing the concept of self-interference that an inband RN may suffer from.
An inband RN operates at the same frequency for a Un interface and a Uu interface. Because the transmitter of the inband RN causes self-interference to the receiver of the inband RN during an inband RN operation as illustrated in FIG. 3, the RN can neither perform downlink reception via the Un interface simultaneously with downlink transmission via the Uu interface nor perform uplink transmission via the Uu interface simultaneously with uplink reception via the Un interface. For example, when the RN transmits data to a UE via a downlink Un interface, its receiver suffers from self-interference caused by the downlink transmission to the UE. Therefore, the RN cannot receive data normally from the DeNB via the downlink Un interface, thereby losing data.